In London, a word used to describe an invitation-only party, where partygoers dress up according to various themes and make every effort to enjoy themselves as a collective. Similar to a rave, but smaller crowd, more exclusive locations, and more interesting people.

We went to a wild-in this Saturday, where everybody was dressed as pimps and strippers and people were having sex on the couches, while the DJ was pumping wicked house tunes.